1. Field of the Invention
This invention relates to an electrical control circuit and more particularly to an electrical control circuit for use with a thermoelectric cooler.
2. Description of the Prior Art
Many different types of control circuits have been designed for regulating power applied to electric equipment. However, unique problems and constraints have been encountered in the area of thermoelectric coolers for use with laser diodes to maintain constant operating temperatures. The system requirements include a control circuit that drives a thermoelectric cooler and maintains a laser diode load at a preset temperature within tolerances of, for example, one-half degree centigrade in the presence of ambient temperature variations ranging between -55.degree. C. to +100.degree. C. Other design constraints include compatibility of the control circuit hybridization and the achievement of maximum circuit efficiency to minimize power consumption. One possible solution to these problems to use of a series pass transistor to control the thermoelectric cooler load current which would be driven by a suitable amplifier with a thermistor sensor at the load for temperature feedback and a DC proportional controller for series pass transistor control. The difficulty with this approach is that the efficiency of a DC proportional controller is of the order of only 50 percent due to losses in the series pass output transistor. Since the thermoelectric cooler can require a large amount of power, for example it can require up to 12 Watts of input power under full load conditions, the overall system losses becomes excessive.
The present invention overcomes these difficulties by providing a pulse width modulator controller, switching transistor and a network filter to supply the load current for the thermoelectric cooler. While this approach is more complex than a DC controller it has the advantage of the output transistor operating between a fully-off and a saturated condition and thereby substantially improving circuit efficiency.